Stretch reflex

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The patellar reflex is an example of the stretch reflex. Patellar tendon reflex arc.png
The patellar reflex is an example of the stretch reflex.

The stretch reflex (myotatic reflex), or more accurately "muscle stretch reflex", is a muscle contraction in response to stretching a muscle. The function of the reflex is generally thought be maintaining the muscle at a constant length but the response is often coordinated across multiple muscles and even joints. [1] The older term deep tendon reflex is now criticized as misleading. Tendons have little to do with the response, and some muscles with stretch reflexes have no tendons. Rather, muscle spindles detect a stretch and convey the information to the central nervous system. [2]

Contents

As an example of a spinal reflex, it results in a fast response that involves an afferent signal into the spinal cord and an efferent signal out to the muscle. The stretch reflex can be a monosynaptic reflex which provides automatic regulation of skeletal muscle length, whereby the signal entering the spinal cord arises from a change in muscle length or velocity. It can also include a polysynaptic component, as in the tonic stretch reflex. [3]

When a muscle lengthens, the muscle spindle is stretched and its nerve activity increases. This increases alpha motor neuron activity, causing the muscle fibers to contract and thus resist the stretching. A secondary set of neurons also causes the opposing muscle to relax.

Gamma motoneurons regulate how sensitive the stretch reflex is by tightening or relaxing the fibers within the spindle. There are several theories as to what may trigger gamma motoneurons to increase the reflex's sensitivity. For example, alpha-gamma co-activation might keep the spindles taut when a muscle is contracted, preserving stretch reflex sensitivity even as the muscle fibers become shorter. Otherwise the spindles would become slack and the reflex would cease to function.

This reflex has the shortest latency of all spinal reflexes including the Golgi tendon reflex and reflexes mediated by pain and cutaneous receptors. [4]

Structures

The stretch reflex is accomplished through several different structures. In the muscle, there are muscle spindles, whose intrafusal muscle fibers lie parallel to the muscle and sense changes in length and velocity. The afferent sensory neuron is the structure that carries the signal from the muscle to the spinal cord. It carries this action potential to the dorsal root ganglion of the spinal cord. The efferent motor neuron is the structure that carries the signal from the spinal cord back to the muscle. It carries the action potential from the ventral root of the spinal cord to the muscle down the alpha motor neuron. [5] This synapses on the first structure discussed, the extrafusal fibers of the muscle spindle.

Examples

A person standing upright begins to lean to one side. The postural muscles that are closely connected to the vertebral column on the opposite side will stretch. The muscle spindles in those muscles will detect this stretching, and the stretched muscles will contract to correct posture.

Other examples (followed by involved spinal nerves) are responses to stretch created by a blow upon a muscle tendon:

Another example is the group of sensory fibers in the calf muscle, which synapse with motor neurons innervating muscle fibers in the same muscle. A sudden stretch, such as tapping the Achilles' tendon, causes a reflex contraction in the muscle as the spindles sense the stretch and send an action potential to the motor neurons which then cause the muscle to contract; this particular reflex causes a contraction in the soleus-gastrocnemius group of muscles. Like the patellar reflex, this reflex can be enhanced by the Jendrassik maneuver.

Spinal control

Spinal control of the stretch reflex means the signal travels between the muscle and spinal cord. The signal returns to the muscle from the same spinal cord segment as where it entered the spinal cord. This is the shortest distance for a reflex signal to travel, thus creating a fast response. These responses are often referred to as short latency stretch reflexes. [6]

Supraspinal control

Supraspinal control of the stretch reflex means the signal travels above the spinal cord before traveling back down to the same segment it entered the spinal cord from. The responses from these pathways are often termed medium or long latency stretch reflexes, because the time course is longer due to distance it needs to travel. [7] The central nervous system can influence the stretch reflex via the gamma motoneurons, which as described above control the sensitivity of the reflex.

Clinical significance

The patellar reflex (knee jerk) is an example of the stretch reflex and it is used to determine the sensitivity of the stretch reflex. Reflexes can be tested as part of a neurological examination, often if there is an injury to the central nervous system. To test the reflex, the muscle should be in a neutral position. The muscle being tested needs to be flexed for the clinician to locate the tendon. After the muscle is relaxed, the clinician strikes the tendon. The response should be contraction of the muscle. If this is the knee jerk reflex, the clinician should observe a kick. The clinician rates the response. [8]

Grading of stretch reflexes upon tapping muscle tendon [9]
GradeResponseSignificance
0no responsealways abnormal
1+slight but definitely present responsemay or may not be normal
2+brisk physiologic responsenormal
3+very brisk responsemay or may not be normal
4+ clonus always abnormal

The clasp-knife response is a stretch reflex with a rapid decrease in resistance when attempting to flex a joint. It is one of the characteristic responses of an upper motor neuron lesion. [10]

See also

Related Research Articles

Spasticity is a feature of altered skeletal muscle performance with a combination of paralysis, increased tendon reflex activity, and hypertonia. It is also colloquially referred to as an unusual "tightness", stiffness, or "pull" of muscles.

<span class="mw-page-title-main">Motor neuron</span> Nerve cell sending impulse to muscle

A motor neuron is a neuron whose cell body is located in the motor cortex, brainstem or the spinal cord, and whose axon (fiber) projects to the spinal cord or outside of the spinal cord to directly or indirectly control effector organs, mainly muscles and glands. There are two types of motor neuron – upper motor neurons and lower motor neurons. Axons from upper motor neurons synapse onto interneurons in the spinal cord and occasionally directly onto lower motor neurons. The axons from the lower motor neurons are efferent nerve fibers that carry signals from the spinal cord to the effectors. Types of lower motor neurons are alpha motor neurons, beta motor neurons, and gamma motor neurons.

In biology, a reflex, or reflex action, is an involuntary, unplanned sequence or action and nearly instantaneous response to a stimulus.

<span class="mw-page-title-main">Somatic nervous system</span> Part of the peripheral nervous system

The somatic nervous system (SNS) is made up of nerves that link the brain and spinal cord to voluntary or skeletal muscles that are under conscious control as well as to skin sensory receptors. Specialized nerve fiber ends called sensory receptors are responsible for detecting information within and outside of the body.

<span class="mw-page-title-main">Muscle spindle</span> Innervated muscle structure involved in reflex actions and proprioception

Muscle spindles are stretch receptors within the body of a skeletal muscle that primarily detect changes in the length of the muscle. They convey length information to the central nervous system via afferent nerve fibers. This information can be processed by the brain as proprioception. The responses of muscle spindles to changes in length also play an important role in regulating the contraction of muscles, for example, by activating motor neurons via the stretch reflex to resist muscle stretch.

<span class="mw-page-title-main">Reflex arc</span> Neural pathway which controls a reflex

A reflex arc is a neural pathway that controls a reflex. In vertebrates, most sensory neurons do not pass directly into the brain, but synapse in the spinal cord. This allows for faster reflex actions to occur by activating spinal motor neurons without the delay of routing signals through the brain. The brain will receive the input while the reflex is being carried out and the analysis of the signal takes place after the reflex action.

<span class="mw-page-title-main">Type Ia sensory fiber</span> Type of afferent nerve fiber

A type Ia sensory fiber, or a primary afferent fiber is a type of afferent nerve fiber. It is the sensory fiber of a stretch receptor called the muscle spindle found in muscles, which constantly monitors the rate at which a muscle stretch changes. The information carried by type Ia fibers contributes to the sense of proprioception.

<span class="mw-page-title-main">Patellar reflex</span> Monosynaptic reflex

The patellar reflex, also called the knee reflex or knee-jerk, is a stretch reflex which tests the L2, L3, and L4 segments of the spinal cord. Many animals, most significantly humans, have been seen to have the patellar reflex, including dogs, cats, horses, and other mammalian species.

<span class="mw-page-title-main">Gamma motor neuron</span>

A gamma motor neuron, also called gamma motoneuron, or fusimotor neuron, is a type of lower motor neuron that takes part in the process of muscle contraction, and represents about 30% of (Aγ) fibers going to the muscle. Like alpha motor neurons, their cell bodies are located in the anterior grey column of the spinal cord. They receive input from the reticular formation of the pons in the brainstem. Their axons are smaller than those of the alpha motor neurons, with a diameter of only 5 μm. Unlike the alpha motor neurons, gamma motor neurons do not directly adjust the lengthening or shortening of muscles. However, their role is important in keeping muscle spindles taut, thereby allowing the continued firing of alpha neurons, leading to muscle contraction. These neurons also play a role in adjusting the sensitivity of muscle spindles.

Tendon reflex may refer to:

<span class="mw-page-title-main">Alpha motor neuron</span>

Alpha (α) motor neurons (also called alpha motoneurons), are large, multipolar lower motor neurons of the brainstem and spinal cord. They innervate extrafusal muscle fibers of skeletal muscle and are directly responsible for initiating their contraction. Alpha motor neurons are distinct from gamma motor neurons, which innervate intrafusal muscle fibers of muscle spindles.

Biceps reflex is a deep tendon reflex (DTR) test that examines the function of the C5 reflex arc and the C6 reflex arc. The test is performed by using a tendon hammer to quickly depress the biceps brachii tendon as it passes through the cubital fossa. Specifically, the test activates the stretch receptors inside the biceps brachii muscle which communicates mainly with the C5 spinal nerve and partially with the C6 spinal nerve to induce a reflex contraction of the biceps muscle and jerk of the forearm.

The triceps reflex, a deep tendon reflex, is a reflex that elicits involuntary contraction of the triceps brachii muscle. It is sensed and transmitted by the radial nerve. The reflex is tested as part of the neurological examination to assess the sensory and motor pathways within the C7 and C8 spinal nerves.

Hoffmann's reflex is a neurological examination finding elicited by a reflex test which can help verify the presence or absence of issues arising from the corticospinal tract. It is named after neurologist Johann Hoffmann. Usually considered a pathological reflex in a clinical setting, the Hoffmann's reflex has also been used as a measure of spinal reflex processing (adaptation) in response to exercise training.

<span class="mw-page-title-main">Mesencephalic nucleus of trigeminal nerve</span>

The mesencephalic nucleus of trigeminal nerve is one of the sensory nuclei of the trigeminal nerve. It is located in the brainstem. It receives proprioceptive sensory information from the muscles of mastication and other muscles of the head and neck. It is involved in processing information about the position of the jaw/teeth. It is functionally responsible for preventing excessive biting that may damage the dentition, regulating tooth pain perception, and mediating the jaw jerk reflex.

The Golgi tendon reflex (also called inverse stretch reflex, autogenic inhibition, tendon reflex) is an inhibitory effect on the muscle resulting from the muscle tension stimulating Golgi tendon organs (GTO) of the muscle, and hence it is self-induced. The reflex arc is a negative feedback mechanism preventing too much tension on the muscle and tendon. When the tension is extreme, the inhibition can be so great it overcomes the excitatory effects on the muscle's alpha motoneurons causing the muscle to suddenly relax. This reflex is also called the inverse myotatic reflex, because it is the inverse of the stretch reflex.

Uwe Windhorst is a German neuroscientist, systems scientist and cyberneticist, who was born in Bremen, Germany in 1946. Windhorst became known for his pioneer research in the use of diverse methods of correlation, spectral analysis as well as nonlinear systems analysis to describe the dynamic properties of signal transmission through small neuronal networks assessed in experimental animals.

<span class="mw-page-title-main">Spinal interneuron</span> Interneuron relaying signals between sensory and motor neurons in the spinal cord

A spinal interneuron, found in the spinal cord, relays signals between (afferent) sensory neurons, and (efferent) motor neurons. Different classes of spinal interneurons are involved in the process of sensory-motor integration. Most interneurons are found in the grey column, a region of grey matter in the spinal cord.

<span class="mw-page-title-main">Golgi tendon organ</span> Proprioceptive sensory receptor organ that senses changes in muscle tension

The Golgi tendon organ (GTO) is a proprioceptor – a type of sensory receptor that senses changes in muscle tension. It lies at the interface between a muscle and its tendon known as the musculotendinous junction also known as the myotendinous junction. It provides the sensory component of the Golgi tendon reflex.

Proprioception refers to the sensory information relayed from muscles, tendons, and skin that allows for the perception of the body in space. This feedback allows for more fine control of movement. In the brain, proprioceptive integration occurs in the somatosensory cortex, and motor commands are generated in the motor cortex. In the spinal cord, sensory and motor signals are integrated and modulated by motor neuron pools called central pattern generators (CPGs). At the base level, sensory input is relayed by muscle spindles in the muscle and Golgi tendon organs (GTOs) in tendons, alongside cutaneous sensors in the skin.

References

  1. ↑ Reschechtko S; Pruszynski JA (2020). "Stretch Reflexes". Curr Biol. 30 (18): R1025–R1030. doi: 10.1016/j.cub.2020.07.092 . PMID   32961152.
  2. ↑ Evidence-Based Physical Diagnosis; McGee; Chapter 63. 2018
  3. ↑ Neilson PD (December 1972). "Interaction between voluntary contraction and tonic stretch reflex transmission in normal and spastic patients". J Neurol Neurosurg Psychiatry. 35 (6): 853–60. doi: 10.1136/jnnp.35.6.853 . PMC   494192 . PMID   4346023.
  4. ↑ Spirduso, Waneen Wyrick (1978). "Hemispheric Lateralization and Orientation in Compensatory and Voluntary Movement". Information Processing in Motor Control and Learning. pp. 289–309. doi:10.1016/B978-0-12-665960-3.50019-0. ISBN   9780126659603.
  5. ↑ Dolbow, James; Bordoni, Bruno (2019), "Neuroanatomy, Spinal Cord Myotatic Reflex", StatPearls, StatPearls Publishing, PMID   31869093 , retrieved 2019-12-30
  6. ↑ Feher, Joseph (2012). "Spinal Reflexes". Quantitative Human Physiology. pp. 332–340. doi:10.1016/B978-0-12-382163-8.00036-0. ISBN   9780123821638.
  7. ↑ Eldred E; Granit R; Merton PA (1953). "Supraspinal control of the muscle spindles and its significance". J Physiol. 122 (3): 498–523. doi:10.1113/jphysiol.1953.sp005017. PMC   1366137 . PMID   13118557.
  8. ↑ Walkowski, A. D.; Munakomi, S. (2019). "Monosynaptic Reflex". StatPearls. PMID   31082072.
  9. ↑ Walker, H. K. (1990). Walker, H. K.; Hall, W. D.; Hurst, J. W. (eds.). "Deep Tendon Reflexes". Clinical Methods: The History, Physical, and Laboratory Examinations. PMID   21250237.
  10. ↑ Ashby Pl; Mailis Al; Hunter J (1987). "The evaluation of "spasticity"". Can J Neurol Sci. 14 (3 Suppl): 497–500. doi: 10.1017/s0317167100037987 . PMID   3315151.
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